In many situations where water or other liquids are used domestically or industrially, it is important to quickly detect and limit leakage. For example, when water begins to leak from a water heater tank, air conditioning water supply, or other plumbing system, it is important to have an early warning sounded and for the water supply to the leaking system to be quickly shut off. An extended period of leakage may cause substantial damage to carpeting and flooring, as well as ceilings and walls, if leakage should occur on an upper floor of a multi-story structure, for example.
A system to automatically detect leaks, activate an alarm, and shut off the supply to the leaking system is desirable and preferably should contain its own electrical power supply. This permits the system to remain operable if flooding or other circumstances cause a loss of electrical power. The use of a self-contained power supply requires that the system have an extremely low current drain in both quiescent and operative modes so that the battery will not need frequent replacement.
Such a system can include a probe or sensor which may be placed around the base of a water heater tank or other plumbing fixture to be monitored. The sensor should be able to quickly and accurately sense the presence of surface water anywhere along its length. The sensor also should not be activated by humidity. Some prior art sensors require a large buildup of surface water before they are activated; and in some sensors water can run along the length of the sensor and still not be detected.
The prior art includes a variety of patents relating to leak detection and shut-off systems. U.S. Pat. No. 4,319,232 to Westphal, et al. discloses a liquid detection device having an elongated water sensor. The sensor has two electrical conductors covered by an insulator having a plurality of openings along its length, periodically exposing each conductor to the environment. Exposure of the sensor to a sufficient amount of water to cover two adjacent openings will produce a change of electrical resistance between the conductors. Considerable water buildup around adjacent probe openings is required to generate the change of electrical resistance between conductors needed to activate the alarm. The sensor also requires precise placement in order to detect surface water. Self-powered detecting circuitry in the Westphal et al. device activates an audible alarm and a non-latching switch, which may be used to close an externally-powered fluid control valve. The device will not operate to control externally powered circuitry if external power is lost.
U.S. Pat. No. 4,297,686 to Tom discloses a combined extended probe liquid detection and shut-off system. A solenoid type shut-off valve is used. This type of valve is typically unsuitable for long term, unattended applications due to the substantial current drain required by this type of shut-off valve. Further, the extended probe requires a slow, steady stream of water to activate the alarm. The probe is not sensitive to small drops of surface water which may be an early warning of a leak.
Shut-off valves requiring lower current drain than solenoid valves are known in the prior art. For instance, a valve control member constructed of a shape-memory material is used to control a valve. When the control member is exposed to external heat above its transition temperature, the control member reverts to a prior shape, changing the position of the valve. U.S. Pat. No. 3,613,732 to Willson, et al. discloses several embodiments of this type of valve. The valves disclosed by Willson et al. are limited in the amount of closing force produced by the control member, making them unusable as water shut-off valves in home and industrial plumbing systems where water pressures may exceed 100 lbs/in.sup.2.
Thus, there is a need for an improved water detection sensor which is highly sensitive to the presence of relatively small amounts of surface water in the area being monitored, without causing false alarms because of sensitivity to humidity or dampness. There is also a need for an improved battery-operated system for detecting the presence of water, sounding an alarm, and shutting off a water supply without substantial electrical current drain or the other problems and drawbacks discussed above.